This invention relates to a vehicle with a steerable wheelset. Whilst the invention is primarily described with an embodiment particularly suited for use with Automated Guideway Transit (AGT) systems of the type which use small, individual vehicles, capable of operating at high speeds, the present invention is also suitable for use with a variety of other rail or guideway systems.
There are a number of known vehicles adapted to travel on rail or guideway systems which have steerable wheelsets.
One such system is disclosed in U.S. Pat. No. 4,982,671 (Chollet et al), and relates to a track guided vehicle. Such a vehicle is supported on bogies, where each bogie contains two wheelsets. Magnetic (or other) sensors are used to detect the lateral position of the bogie with respect to the track on which it is running. At least one sensor detects the angle between the two wheelsets. The two wheelsets are connected via linkages and actuators, such that the angle between the wheelsets can be altered to steer the bogie. A servo-control circuit receives signals from the sensors and controls the actuators to steer the wheelsets in response to the detected lateral position of the bogie.
Another known system is disclosed in European Patent 374,290 (Girod et al), and relates to a track guided vehicle. Such a vehicle comprises four wheels that can be independently steered. Laser sensors, located at the front and rear of the vehicle, are used to detect the difference between the track centreline and the vehicle longitudinal axis. A servo-control mechanism controls the steering actuators in order to steer the wheels in response to the sensed signals.
A disadvantage of both of these arrangements is that the lateral forces at the wheel-rail contact zone must serve a dual function, namely to steer the bogie and to oppose any lateral force, such as the centrifugal force experienced by a vehicle while cornering. Consequently the force available for steering the bogie is limited to the difference between the total available force and that already being used to oppose any external lateral forces. In a rail application where a steel wheel rolls on a steel rail, the total available force may be very low. This available force may be substantially required to react centrifugal force, with very little remaining force available to steer the wheelset leading to frequent contact between the wheel flanges and the rails.
A further known system is disclosed in U.S. Pat. No. 5,730,064 (Bishop), and relates to a self-steering bogie for track guided vehicle. The wheelsets are arranged such that a curvature in the rail generates a twist angle between the two wheelsets in the bogie when viewed in end elevation. The mechanism connecting the two wheelsets is arranged so as to steer the wheelsets, in response to rail curvature. A disadvantage of this arrangement when applied to small vehicle guideway systems, which typically use much sharper curves than normal rail systems, is the steer error resulting from twist angle supplied by rapidly changing superelevation. This may add to or subtract from the ideal steering angle required, causing the wheelset to deviate from its idealised path.
Preferably the present invention overcomes the above mentioned disadvantages by providing a vehicle with a steerable wheelset in which the effect of lateral or disturbing forces on the vehicle is minimised.
In one aspect the present invention is a vehicle with at least one steerable wheelset adapted to run on a guideway having two primary running faces laterally offset about a guideway centreline, the wheelset comprising a pair of wheels, each wheel located on opposite sides of the wheelset adapted to engage with a respective one of the two primary running faces, the vehicle further comprising sensing means for sensing lateral displacement of the wheelset with respect to a longitudinally disposed reference path, the sensing means producing a signal for a control system operably connected to an actuating means to steer the wheels in response to the sensed lateral displacement, characterised in that the axes of rotation of the wheels and the primary running faces are inclined downwardly towards the guideway centreline.
In a first embodiment each wheel exerts an engagement force with its respective primary running face, the engagement force on each wheel comprising a perpendicular component to its respective primary running face and a parallel component to its respective primary running face substantially transverse to the guideway centreline, wherein horizontal forces acting on the wheelset substantially perpendicular to the guideway centreline are substantially resisted by the sum of of the horizontal vectors of the perpendicular components.
In a second embodiment embodiment each wheel exerts an engagement force with its respective primary running face at a contact zone, the engagement force on each wheel comprising a first component perpendicular to its respective primary running face and a second component parallel to its respective primary running face substantially transverse to the guideway centreline, wherein a first plane perpendicular to the axis of rotation of one of the wheels passes through its respective contact zone, and a second plane perpendicular to the axis of rotation of the other wheel passes through its respective contact zone, the first and second planes intersecting along an intersection line disposed above and between the wheels, wherein horizontal forces acting on the wheelset substantially transverse to the guideway centreline at or near the intersection line are substantially resisted by perpendicular components of the engagement forces acting at the primary running faces, such that substantially all of the parallel components of the engagement forces acting at the primary running faces are available to steer the wheelset.
Preferably the intersection line passes through the centre of gravity of vehicle.
It is preferred that the sensing means comprises at least one sensor located either ahead or behind the wheelset, or laterally offset with the wheelset. Alternatively the sensing means comprises at least two sensors, one of which is located ahead of the wheelset and the other is located behind the wheelset.
It is preferred that the longitudinally disposed reference path is substantially contiguous with the guideway centreline.
Alternatively, it is preferred that the longitudinally disposed reference path is substantially parallel to, but laterally offset from the guideway centreline.
It is preferred that a secondary running face lies immediately adjacent to, and substantially parallel to, at least one primary running face.
It is preferred that the longitudinally disposed reference path is contiguous with the second running face,
Alternatively, it is preferred that a secondary running face lies immediately adjacent to and substantially parallel to each primary running face and the longitudinally disposed reference path is contiguous with the lateral centreline between the respective two secondary running faces.
It is preferred that at least one of the wheels also incorporates a flange, adapted to engage with the secondary running face.
It is preferred that the control system calculates a virtual longitudinally disposed reference path which is not necessarily parallel or contiguous with the guideway centreline.